ABSTRACT Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a familial disorder in which polymorphic ventricular arrhythmias are caused by intensely-felt emotion or physical exertion. CPVT results from excess diastolic calcium leak and carries high risk of sudden cardiac death. On the basis of family studies, most cases of CPVT are associated with mutations in the cardiac ryanodine receptor (RYR2), the channel by which calcium exits the sarcoplasmic reticulum (SR), or cardiac calsequestrin (CASQ2), the principal calcium store of the SR. While RYR2-associated CPVT clearly exhibits autosomal dominant inheritance, the mode of inheritance of CASQ2-associated CPVT presents a puzzle. Many of the known CPVT-causing mutations appear to require recessive inheritance for penetrant disease, despite the fact that calsequestrin, a protein that must dimerize and then oligomerize to perform its function, would classically be considered vulnerable to functional disruption by heterozygous missense mutations. It is not known why some CASQ2 mutations, such as the recently reported K180R substitution, clearly have dominant deleterious effect, while others are pathogenic only when carried recessively. We propose to resolve this conundrum via a thorough structural and biophysical study of the cardiac calsequetrin ?lament. We have succeeded in determining what we believe is the ?rst credible x-ray structure of the cardiac calsequestrin ?lament, revealing extensive buried surface area at oligomer contacts, as well as the ?lament's helical turn. We ?rst hypothesize that the oligomer forms from a series of calcium salt bridges, such that mutations at calcium-binding sites make the oligomer particularly unstable. In our ?rst aim, we propose to use x-ray crystallography with isomorphous calcium replacement to comprehensively map the calcium-binding sites of the calsequestrin ?lament, show that several known disease mutations are at ligand sites, and reveal that the oligomer requires calcium coordination in order to form. We also hypothesize that oligomer formation stabilizes dimers via extended all-by-all interactions, so that mutations that destabilize the calsequestrin dimer interface, when carried only in one copy, are rescued by the inherent stability of the oligomer at elevated SR-like calcium levels. In our second aim, we propose to demonstrate this rescue effect using biochemical assays and molecular dynamics simulations. In our third aim, we propose to model CPVT-causing CASQ2 mutations in iPS cells and demonstrate using a largely cell autonomous disease phenotype that the mutations that we have predicted on the basis of structure and dynamics to act in dominant fashion are in fact the ones that do so. The result of this project will be a new classi?cation of CASQ2 mutations, an understanding of which types of mutations (oligomer interface vs dimer interface) have which disease inheritance mode and why, and a much improved basis for helping clinicians and patients decide whether aggressive therapy - which carries its own risks and morbidities - is warranted.